This year in Astana colours, Rosa was pipped on the line by Esteban Chaves (Orica-BikeExchange) at Il Lombardia, finished 10th at Liege-Bastogne-Liege, won the mountain classification and stage 5 at the Tour of the Basque Country, and finished eighth on GC at the Criterium du Dauphine.
Exciting finish at the 2016 Il Lombardia
More importantly, the 27-year-old Italian has served as a loyal lieutenant at all three Grand Tours throughout his career.
"Joining Team Sky is a dream come true and I'm very excited to pull on the jersey," Rosa said.
"It's a big step forward in my cycling career and I'm sure Team Sky is the best environment for me to grow and mature as a rider.
"My ambitions are in-line with those of the team: to work hard together to achieve important results. I'm ready to support the team's leaders in Grand Tours and do well in the one-day races that suit my skill set."
Rosa's agreement is inked for the end of 2019.
25-year-old Elissonde joins Sky from FDJ. In the red, white and blue, he came agonisingly close to claiming the 2016 Vuelta a Espana King of the Mountains classification, finishing just one point behind Omar Fraile (Dimension Data). Here, he almost won the queen stage (stage 14) on the summit at Aubisque but was bested by a highly motivated Robert Gesink. Elissonde finished the Spanish Grand Tour with another two, top five stage finishes and 20th on GC.
Elissonde featuring in stage 14 of the 2016 Vuelta a Espana
"Team Sky is one of the best teams in the world for climbers. I'm really happy and excited to be a part of the group next year. It's a really big step in my career," Elissonde said.
"I will try to learn from the team's leaders and be a good team player. I'm still young so it's important for me to learn, do the best I can, and improve at every aspect of the sport.
"I live in Nice and used to train with Ian Boswell and I know [Sport Director] Nicolas Portal a little bit. I spoke with him two years ago so I'm looking forward to getting to know him more.
"It's very cool to be the first French rider on the team for six years. This is a great opportunity for me."
The Frenchman has raced five Grand Tours and won a Vuelta stage in 2013, aged 22. His agreement with Sky runs for two seasons.